1. Field of the Invention
This invention relates to multicomponent solvents formulated for use as cleaning agents and diluents with low or no toxicity and non-flammable or non-combustible characteristics. More particularly, the present invention relates to multicomponent solvents formulated to be miscible with oil-, and resin-based paints, but is not limited thereto. The solvents of the present invention act as cleaners of, and thinners for, such paints, with minimal effect on the desired properties of the paint, while maintaining the noted safety features.
2. Description of The Prior Art
In the field of solvents, and, in particular, paint solvents used to thin or otherwise dilute the paint resin or binder, there has been a long history of use of compounds and compositions with less-than-desirable health and safety characteristics. Of course, the hazards of such solvents were not well known and in any case, suitable, inexpensive alternatives were few and far between. The situation remains basically the same today. As a result, solvents that were used long ago continue to remain in use. Specifically, there exists in primary use today thinning compounds known generically as turpentine and "mineral spirits." Both compounds have desirable properties in that they are miscible with many commercially-available oil-based paints and they are relatively non-hazardous. That characterization is of course dependent upon comparing those solvents with suitable alternatives.
There are several methods used to define a satisfactory solvent. Two of the more widely-used techniques include the Kauri-butanol test, conducted under American Society of Testing and Materials (ASTM) D-1133, and a Solubility Parameter evaluation. The Kauri-butanol test provides a relative measure of the solvency of a compound or mixture when that compound or mixture is added to a standard solution of kauri resin in butanol. For the most part, the higher the value, the greater the solvating capacity of the solvent. In general, a kauri-butanol (K.sub.B) value of less than 32 indicates a compound with poor solvency characteristics, at least for oil-based resins, while a K.sub.B value above 90 indicates good solvency for such resins.
The measure of the solvency characteristics of a compound or mixture under the solubility parameter evaluation is actually a determination based upon the inherent characteristics of the material rather than its interaction with another compound. The solubility parameter is based on the heat of vaporization of the material, a property available in the appropriate technical literature for many solvents. This parameter is perhaps more useful in determining the relative solvency of a wider array of compounds than is the kauri-butanol test. Under this measure, a high value tends to indicate a solvent suitable for use with polar compounds, while a lower number tends to indicate that the solvent can be used to dilute non-polar compounds. Water, for example, has a solubility parameter of 23.4, while mineral spirits has a value of 6.9.
Another measure of the suitability of a particular solvent with a particular resin is its evaporation rate. This rate determines the speed with which a solvent-thinned paint, for example, will dry after application. Depending upon the desired drying rate for the resin, one solvent may be more useful than another. In general, a highly-evaporative solvent will produce faster resin drying. Of course, that drying rate is a function of temperature. It should also be noted that those solvents with relatively high evaporation rates, such as mineral spirits, also tend to have lower flash points and therefore present flammability hazards.
Of the two paint solvents most commonly used, turpentine and mineral spirits, turpentine was the first one in wide use. Turpentine is an essential oil of the naturally-occurring terpene family, consisting principally of alpha-pinene. Broadly stated, its characteristics include: 1) good miscibility with an array of commonly-used paint resins; 2) a suitable evaporation rate, providing sufficient time to easily apply the resin and a reasonable drying time; 3) a flash point (the temperature at which the vapor space just above the compound will ignite) that is higher than offer available solvents; and 4) an odor that is not completely unpleasant. However, in spite of the noted advantages, there are problems with turpentine, including its flash point (only about 90.degree. F.) and an undesirable level of toxicity, that have made the search for substitutes an urgent one.
For a number of years mineral spirits have been used as substitutes for turpentine-primarily on the basis of lower cost and greater availability, rather than on any improvement in flash point or toxicity. The solubility parameter associated with mineral spirits is slightly less than that of turpentine (6.9 versus 8.1) though well within the values evidencing suitable solubility for oil-based resins. Mineral spirits (or white spirits) is a term used to designate a wide class of petroleum-based aliphatic-hydrocarbon solvents in the C10-C15 range that boil at temperatures greater than 300.degree. F. Due to the abundance of petroleum, lower-priced mineral spirits have, to a great extent, replaced turpentine as oil-based-paint solvents. Of course, in the generic class of petroleum-based solvents there are a range of compounds with varying characteristics. In spite of their noted usefulness in providing good thinning for oil-based paints, there is a continuing push to reduce direct human contact with hydrocarbons generally, given recent observations that such compounds tend to be carcinogenic.
In addition to mineral spirits, there are the lighter aliphatics, such as the hexanes and octanes that evaporate more quickly and/or are more compatible with particular resins. To a certain extent, these compounds may make up a fraction of the generic mineral spirits solvent. However, in some specific cases they may be used directly as solvents in and of themselves. A wide range of aromatic hydrocarbons, including toluene and xylene, are also considered useful solvents, particularly for oil-based paints that have aromatics as constituents. Of course, the hazards associated with these aromatics are at least as well-known as those observed with the other hydrocarbons. In fact, enactment of the Clean Air Act of 1990 has restricted the use of such solvents, among many others. As a result, they are little used as thinners or cleaners by private individuals. Nevertheless, on occasion, they do make up at least a small portion of the mixtures used as thinners, including those designated as mineral spirits.
In spite of the well-known hazards associated with the aliphatic and aromatic hydrocarbon solvents, including their relatively-low flash points, they continue to be used as thinners and cleaners for a variety of paints. Of course, these solvents, along with turpentine, are inadequate for paints that are not oil- or resin-based--typically the cellulose-derived materials known generically as lacquers (resins derived from plants and fossilized material). As a result, other solvents are used to perform the same functions undertaken by the noted compounds and mixtures. For example, the solvents used to thin shellacs (insect-produced resins) are alcohol-based, the solvents commonly used to thin nitrocellulosic substances are esters. Ketones are often used as solvents for acrylic and vinyl resins. Again, while useful in thinning and cleaning a range of resins, these various oxygenated solvents suffer the same deficiencies associated with turpentine, mineral spirits, and the other hydrocarbon solvents. Those deficiencies are, primarily, unacceptable levels of toxicity and low flash points. It has been observed that mixing relatively dissimilar solvents, such as combining a hydrocarbon with an oxygenated solvent, can result in a single solvent mixture with average solubility and evaporation rate, useable with dissimilar resin types. However, such mixtures may still suffer the deficiencies of the individual constituents.
As noted, one principal concern with a wide array of solvents involves the possibility that the vapor space above the particular liquid will ignite under certain conditions. In order to support such ignition, a sufficient quantity of the solvent's vapor must be present in combination with oxygen. One measure used to define the temperature at which a combustible mixture of oxygen with the vapor of a particular solvent will support ignition is the flash point. The flash point is the temperature at which the solvent's vapor pressure is sufficient to produce a flame in the present of an ignitor. Low flash points generally indicate a greater likelihood that ignition will occur. Generally, they also indicate that evaporation will occur sooner than with high-flash-point solvents. Under U.S. Department of Transportation standards, a solvent is considered: 1) flammable if it has a flash point below 135.degree. F.; 2) non-flammable, but combustible if it has a flash point in the range 135.degree. F.-175.degree. F.; and 3) non-combustible if it has a flash point greater than 175.degree. F. In simple terms under these definitions, a flammable compound is one that can ignite under ambient conditions that can occur naturally and that are to be expected during transport, while a combustible compound is one that can ignite under unlikely but possible transportation conditions. There are some suitable oil-based solvents that can be considered non-flammable, such as heavy aliphatic- and aromatic-hydrocarbons. Even fewer can be considered non-combustible.
One attempt at fixing the flammability problem noted for many solvents involved the use of chlorinated and/or fluorinated hydrocarbons either as substitutes for those earlier solvents, or as constituents in mixtures containing the earlier solvents. The high flash points of these halogenated hydrocarbons, which include methylene chloride and trichloroethylene, among others, translate into a significant reduction in the flammability of the solvent. However, the commonly-used halogenated hydrocarbons are generally considered to be more toxic than the earlier solvents. In fact, many are deemed to be carcinogenic. Moreover, the deleterious affect of the highly-volatile chlorinated solvents, and chlorofluorocarbons in particular, on the ozone layer is well documented.
Apart from this failed attempt to provide solvents with both good solvating characteristics for resins of interest and low-, or non-flammability, the toxicity of such compounds has always been considered at first an unknown, and then later, a necessary evil. Toxicities of many substances, including solvents, are most commonly defined either by their Threshold Limit Value (TLV) or by LD50 Test results. TLV is the quantity of a compound that a person can be exposed to over a given period of time without adverse health effects. The time period is typically defined as an 8-hour work day and the values are measured in parts of the vapor of the compound per million parts of air containing the vapor, at standard temperature and pressure. Under Occupational Safety & Health Administration regulations, a low TLV number (e.g., less than 100 ppm) indicates a toxic compound, whereas a high TLV (e.g., greater than 100 ppm) indicates reduced toxicity. Turpentine, for example, has a TLV of 100 ppm, while mineral spirits have TLV values higher than that.
The LD50 Test, on the other hand, involves the introduction of the compound into a living organism, by ingestion, inhalation, injection, or the like, and observation of whether death occurs within a specified period of time. While a variety of organisms may be evaluated, one of the more commonly used is a laboratory white rat. Under the Federal Hazardous Substance Act (FHSA), a substance is considered toxic if at least one-half of the rats in a test die within ten days of receiving the substance and they have received at least five grams per kilogram of the animal's body weight. Of course, when at least half die within the noted time period after receiving less than five grams/kilogram, the substance is also deemed to be toxic. If fewer than half the rats die over the ten-day period after receiving at least 5 grams/kilogram, the substance is, by FHSA definition, non-toxic. Typically, the number of rats observed in a test group is ten.
A further characteristic considered to be important by paint thinner users--particularly those users who work in close proximity to the paint source, such as artists, for example--is the odor of the mixture in use. At first that may not seem to be a significant factor, in light of flammability and toxicity concerns. However, when one is exposed to an offensive odor for an extended period of time, that characteristic becomes increasingly significant. Therefore, many prior-art solvents, while effective thinners and cleaning agents, either are simply too offensive, or they require the addition of odor maskers--generally compounds that are compatible with the particular solvent and that also provide a not unpleasant odor. One such substance commonly used in masking is d-limonene, a naturally-occurring compound in the same terpene family as turpentine. It has a lemon odor and solvating characteristics for some natural resins. However, it does have a flash point slightly below that of turpentine.
Therefore, there exists a need for solvents that can be used to improve the applicability, or "spreadability," of resins, and oil-based paints in particular, on surfaces of interest; that is, solvents--as well as solvent mixtures--that can be used to thin such resins without significantly affecting the ultimate properties of the dried resins. Of course, an accompanying feature of such solvents is the ability to use them to clean items in contact with the undried resins when the solvents are used in excess.
Further, there exists a need for such solvents with evaporation rates that are compatible with the drying of the particular resins. In other words, such solvents must not evaporate too quickly so as to cause the resins to crack upon drying; nor must they evaporate too slowly so as to prevent drying of the resins. There also exists a need for such solvents with very little, if any, unpleasant odor emissions, particularly when the resin to be thinned is used in close quarters.
Still further, there exists a need for such solvents with low-, or no-toxicity, as defined under standards such as TLV ratings or LD50 values. Yet further, there exists a need for such solvents with all of the above-noted characteristics as well as having a composition that renders them non-flammable or, ideally, non-combustible.